Commercially, urea is synthesized by reacting ammonia and carbon dioxide. Ammonia is synthesized by reacting nitrogen and hydrogen over a catalyst. The urea synthesis plant and ammonia synthesis plants are frequently situated immediately adjacent each other since the feed gas for both processes is generally obtained from the same source, viz, a gas obtained by the partial oxidation of oil or coal. Typically, such gas comprises (mole %).
______________________________________ Hydrogen 45-70% Carbon Dioxide 25-45% Hydrogen Sulfide 0.5-1.5% Carbonyl Sulfide 0.001-0.2% Carbon Monoxide, Argon, Nitrogen, Balance (typically Methane 0.5-25% ______________________________________
For ammonia synthesis, hydrogen free of sulfur compounds and carbon dioxide is recovered and for urea synthesis carbon dioxide free of sulfur compounds is recovered. Sulfur compounds in ammonia synthesis are prohibited because they act as catalyst poisons. Urea synthesis differs from ammonia synthesis in that hydrocarbons, (CH.sub.4) CO, and particularly H.sub.2, are detrimental when present in the urea carbon dioxide feed gas. These gases tend to "build-up" in the system and, because O.sub.2 is typically added to inhibit urea reaction system corrosion, can present a plant explosion hazard. Accordingly the hydrocarbons, CO and hydrogen are removed from the feed CO.sub.2 by catalytic oxidation prior to introduction to the urea synthesis unit. Sulfur compounds are poisonous to the oxidation catalysts and therefore have to be removed from the system in addition to the hydrocarbons, CO and hydrogen. Note, however, that sulfur components are reacted away and do not build up and cause explosion problems.
Various processes have been utilized to remove acid-gases from gas mixtures so that the poisonous effects on catalysts in both ammonia and urea production can be eliminated. These processes include chemical absorption, physical absorption, and adsorption on solids. Physical absorption processes take advantage of different physical solubilities of gases in liquid and provide significant advantages as compared to other processes in terms of reduced power consumption. A description of physical absorption processes is set forth in the following references:
U.S. Pat. No. 4,050,909 and Ranke, Linde-Report on Science and Technology, Volume 18, pages 7-13 (1973) disclose a process for recovering substantially pure carbon dioxide and hydrogen for use in the production of urea and ammonia respectively from gas obtained by the partial oxidation of oil or coal. In this recovery process the raw gas is initially scrubbed in a bottom portion of an absorption column with carbon dioxide laden methanol at high pressure thereby forming a scrubbing agent laden with sulfur compounds and an initially scrubbed gas. The initially scrubbed gas then is introduced to another section of the absorption column and contacted with pure methanol thereby producing a gas discharge stream enriched in hydrogen (containing small proportions of nitrogen, argon, methane (hydrocarbons), carbon monoxide and carbon dioxide and a carbon dioxide laden methanol stream. A portion of the carbon dioxide laden methanol stream is directed to the initial scrubbing stage while the balance is regenerated via pressure reduction and thermal techniques. Optionally, nitrogen is used to aid in removing carbon dioxide from the solvent. Carbon dioxide free of sulfur compounds is obtained as a product and pure methanol is recovered for recycling to the process.
U.S. Pat. No. 3,498,067 discloses a process similar to the '909 process. A methanol scrubbing agent laden with carbon dioxide is used as a solvent for hydrogen sulfide and residual carbonyl sulfide. By using the partially laden scrubbing agent, H.sub.2 S scrubbing can be confined to a small section in the column and energy requirements are reduced by virtue of passing less solvent through the lower section. U.S. Pat. No. 4,152,129 discloses a process for cryogenically separating carbon dioxide from methane where carbon dioxide is present in large quantities. The process comprises fractionating the carbon dioxide by selecting conditions such that the carbon dioxide will not freeze out in the equipment. Streams rich in carbon dioxide and in methane are obtained.
In the prior art physical absorption processes substantially all of the sulfur compounds, including carbonyl sulfide, were removed from the feed gas in a single absorption step. Each employed a carbon dioxide laden solvent as a scrubbing agent and solvent rates were adjusted so that substantially all of the sulfur compounds were removed in the initial scrubbing operation. Solvent requirements to achieve removal of all the sulfur compounds, and carbonyl sulfide particularly, in the initial stage required high solvent rates and required substantial energy requirements to remove the sulfur compounds from the solvent. This is particularly true with the commercial physical absorption processes using N-methyl pyrrolidone or mixtures of the alkyl ethers of polyethylene glycol as solvents.